Probing the Difference in the Complexation of Trivalent Actinides and Lanthanides with a Tridentate N,O-Hybrid Ligand: Spectroscopy, Thermodynamics, and Coordination Modes

Comparatively revealing the complexation behavior of trivalent actinides and lanthanides with functional ligands in aqueoussolution is of great importance to enrich our knowledge on the fundamentalcoordination chemistry of trivalent f-block elements and to control the fateof minor actinides in nuclear fuel cycles. In this work, the complexation ofAm(III) and Nd(III), representatives for trivalent actinides and lanthanides,respectively, with a N,O-hybrid ligand 6-(dimethylcarbamoyl)picolinic acid(DMAPA, denoted as HL) was investigated by absorption spectroscopy,calorimetry, X-ray crystallography, and density functional theory (DFT)calculations. Successive formation of 1:1, 1:2, and 1:3 (metal/ligand)complexes of Am(III) and Nd(III) with DMAPA was identified, and thecorresponding thermodynamic parameters were determined. The bindingstrength of Am(III) with DMAPA is slightly stronger than that of Nd(III),and the complexation of Nd(III) with DMAPA is mainly entropy-driven.The crystal structure of the 1:2 Nd(III)/DMAPA complex and the DFT calculation shed additional light on the coordination andstructural characteristics of the complexes. In contrast to the Nd-N bond in the Nd(III)/DMAPA complex, the Am-N bond in theAm(III)/DMAPA complex exhibits more covalency, which contributes to the slightly stronger complexation of Am(III) with DMAPA.